As part of a multi-center collaborative study, we measured antibody levels to Haemophilus influenzae type b (Hib) in ten chronic peritoneal dialysis (CPD) patients, aged 39 months or less, who were immunized while on CPD. Nine of the ten developed protective antibody levels to Hib. Four patients had serial measurements of antibody and all maintained protective levels, although the levels did decrease in two patients. Thus most, but not all, infants immunized with Hib vaccine while on CPD develop protective antibody levels. The factors responsible for vaccine failure are not clear. Whether patients maintain protective antibody over time needs to be determined.